The present invention relates to a lead frame used for mounting a semiconductor element in a resin-sealed semiconductor device for a large amount of power.
A conventional resin-sealed semiconductor device for a large amount of power as shown in FIG. 7 comprises an element-mount part 3, a horizontal part 4 for fixing a lead frame for resin sealing and a central lead 5, and these parts (3-5) are formed integrally. A semiconductor element 7 is die-bonded to the lead frame comprising side leads that are formed in parallel on both sides of the central lead 5. The semiconductor element 7 is connected with the side leads 6 by wires 10. The central lead 5 is connected directly with an electrode on the backside of the semiconductor element 7. Alternatively, the central lead 5 can be connected by a wire 10 in the same manner as the side leads 6. The element-mount part 3 and the horizontal part 4 of the lead frame are made to be thicker than the other parts in order to efficiently radiate heat generated during operation of the semiconductor device.
The connected lead frame is sealed with resin to form a semiconductor device for a large amount of power. In this case, a resin thickness t1 from the backside and from the side lead frames should be at least 0.3 mm to provide a withstand voltage of at least 5 KV. In order to protect the wires and the semiconductor, the resin thickness t2 from the surface of the semiconductor element should be at least 3 mm.
For providing better thermal radiation efficiency during operations, a metal plate of a conventional lead frame is as thick as about 1.3 mm at the element-mount part and the horizontal part for positioning (both of which are to be resin-sealed), while the central lead and side leads are relatively thin, for example, about 0.6 mm. A deformed long metal plate having varied thickness entails a much higher cost for processing compared to a flat plate having even thickness. Therefore, a thin and uniform (in thickness) plate can reduce costs for both materials and processing.
However, when a metal flat plate is processed to be a uniform lead frame that is 1 mm or less in thickness, the resin will expand more in the surface than in the backside as shown in FIG. 8(B) due to the heat from the semiconductor element during operations or heat provided during solder reflux, while the surface resin contracts as shown in FIG. 8(C) when the resin is cooled to a room temperature or below, since the sealing resin has a greater thermal expansion coefficient compared to the metal material of the lead frame. Conventionally, distortion of the resin caused by the thermal expansion and contraction does not affect the semiconductor element, since a thick element-mount part is sufficiently strong. For a thin lead frame, the resin becomes thick if the backside of the same resin at the element-mount part has the thickness equal to a conventional backside resin. As a result, the surface resin will be further distorted, the thin element-mount part can be deformed and the semiconductor element can crack.
In addition, since the resin expands considerably due to heat generated by the resin itself or heat from outside, gaps are formed between the lead frame and the resin. This may cause deterioration in properties caused by the invasion of moisture from outside through the gap.
In order to solve the above-mentioned problems, this invention relieves resin distortion in a thin and uniform lead frame caused by the difference in thermal expansion coefficients between the lead frame and the sealing resin after repetitions of heating and cooling during solder reflux and during operations of a power semiconductor element, so that this invention prevents deterioration of the properties of the semiconductor element due to moisture invasion from outside and mechanical pressure caused by a gap formed between the lead frame and the sealing resin.
For this purpose, a lead frame of this invention includes a part for mounting an element of a resin-sealed semiconductor device, a horizontal part for fixing the lead frame for resin sealing, and a central lead formed integrally with the element-mount part and the horizontal part, where side frames are formed in parallel on both sides of the central lead. The element-mount part and the central lead have the same thickness of no more than 1 mm, and at least one pair of resin-anchoring parts are provided on the opposing two sides on the periphery of the element-mount part, and wherein the resin-anchoring parts are notches whose inner portions are wider than the openings.
As a result, resin will cohere in the horizontal direction, and a semiconductor element can be protected from applied mechanical and thermal stress. The moisture resistance is also improved. Thus, the reliability of the semiconductor device can be improved.
Similar effects can be obtained if the resin-anchoring parts are protrusions. Furthermore, the notches are preferably trapezoidal or the like, but the shape is not limited thereto. Any shapes can be selected as long as similar effects can be expected.
It is also preferable that the resin-anchoring parts are formed at four corners on the periphery of the element-mount part. More preferably, the resin-anchoring parts are arranged symmetrically with respect to centerlines of the element-mount part. In this arrangement, the resin-anchoring parts can be placed at substantially the same distance from the center of the element-mount part. So the resin will cohere uniformly, and stress due to thermal expansion or external pressure can be dispersed uniformly.
It is also preferable that the lead frame is bent upward at one side opposing to the side of the element-mount part connected with the central lead, so that a semiconductor device including a lead frame of this invention can be fixed securely to a mount board.
It is also preferable that at least one pair of resin-anchoring parts are formed on both sides of the central lead at one side of the element-mount part to which the central lead is connected, so that the resin will cohere in the length direction (Y direction) as well as the width direction (X direction).
It is also preferable that the central lead connected to the element-mount part is bent upward in the vicinity of the connection, so that deformation and distortion due to the difference in thermal expansion coefficients can be reduced.
It is also preferable that the notches are bent upward, so that the backside resin can be prevented from being thinned at the point.
It is also preferable that the notches formed on one side of the element-mount part have a depth not more than the plate thickness of the element-mount part. If the depth is less than 80% of the plate thickness, cutting out the notches by using a cutting-blade becomes difficult. The depth should not exceed the plate thickness of the element-mount part since excessively deep notches may reduce the element-mount part area.
It is also preferable that the inner sides of the notches formed on one side of the element-mount part are parallel to the element-mount part, so that the resin will cohere and provide a uniform load on the lead frame in order to uniformly disperse stress caused by thermal expansion or external pressure.
A method of producing a lead frame of this invention, that is, a lead frame including a part for mounting an element of a resin-sealed semiconductor device, a horizontal part for fixing the lead frame for resin sealing, and a central lead that is formed integrally with the element-mount part and the horizontal part, where side lead frames are formed in parallel on both sides of the central lead, comprises a process including a step of providing a central lead that has a thickness equal to the element-mount part, and a step of providing at least one pair of resin-anchoring parts on two opposing sides on the periphery of the element-mount part by cutting the element-mount part from the backside.
In the structure, the resin will cohere in the horizontal direction, and the semiconductor element can be protected from applied mechanical or thermal stress. The moisture resistance also is improved. As a result, the reliability of the semiconductor device can be improved.
A semiconductor device of this invention includes a lead frame that is composed of a part for mounting an element of a resin-sealed semiconductor device, a horizontal part for fixing the lead frame for resin sealing, and a central lead that is formed integrally with the element-mount part and the horizontal part, where side lead frames are formed in parallel on both sides of the central lead. The element-mount part and the central lead have the same thickness of no more than 1mm, and the semiconductor device has a lead frame having at least one pair of resin-anchoring parts on the opposing two sides on the periphery of the element-mount part where the resin-anchoring parts are notches whose inner portions are wider than the openings, a semiconductor element mounted on the element-mount part, and a resin envelope for sealing the lead frame and the semiconductor element integrally.
In the structure, the resin will cohere in the horizontal direction in the lead frame, and the semiconductor element can be protected from applied mechanical or thermal stress. The moisture resistance is also improved. As a result, semiconductor devices with high reliability can be provided.
It is preferable for the semiconductor device of this invention that the resin-anchoring parts are arranged symmetrically with respect to the semiconductor element. In this arrangement, the resin-anchoring parts can be placed substantially the same distance from the center of the element-mount part. So the resin will cohere uniformly and stress caused by thermal expansion or external pressure can be dispersed uniformly.